The present invention relates to a monodirectional impeller for centrifugal electric pumps having a permanent-magnet synchronous motor.
It is known that permanent-magnet synchronous electric motors have a general structure which comprises a stator, provided with an electromagnet constituted by a lamination pack and by corresponding windings, and a rotor, which is arranged between two pole shoes formed by the stator and is crossed axially by a shaft which is rotatably connected to a supporting structure.
These motors are bidirectional, i.e., at startup the rotor can be induced equally to turn clockwise or counterclockwise.
This characteristic depends on a plurality of factors, including the arrangement of the polarities of the rotor with respect to the magnetic field generated between the pole shoes of the stator pack when the induction windings are supplied with AC current.
For this reason, permanent-magnet synchronous motors are currently widely used where the direction of rotation is not important; accordingly, for example they are coupled, in centrifugal pumps, to radial-vane impellers which ensure the same performance in both directions of rotation.
In order to increase the efficiency of synchronous-motor electric pumps without resorting to the use of particular electronic starting devices, it is convenient to use vanes which are orientated with a certain curvature profile, which clearly presumes a single direction of rotation of the motor.
Accordingly, electronic starter devices have been devised which guide the motor so that it starts in a single direction of rotation; as an alternative thereto, mechanical devices have been devised which block the rotor when it tends to start in the wrong direction of rotation (reference should be made for example to patent application PD98A000003 of Jan. 8, 1998 in the name of this same Applicant).
In this manner, monodirectional behavior is ensured in any operating condition assumed by the electric pump.
However, the system may generate noise during starting and is a limitation as regards reliability (for high-power pumps), since there is a mechanical device which is subjected to repeated stresses, especially during starting.
A particularly important alternative for a monodirectional synchronous electric pump without mechanical devices for stopping the rotor and without electronic devices (which are reliable but expensive) is constituted by what is disclosed in patent application PD98A000058 of Mar. 19, 1998 in the name of this same Applicant.
This patent application discloses a device which is able to start, with limited power levels, loads which have high moments of inertia, such as impellers with orientated vanes of a centrifugal pump.
In particular, this is a driving device with a larger angle of free rotation between the rotor and the impeller, so as to obtain, with respect to conventional mechanical couplings, several advantages:
reduction of the starting torque for starting the motor;
a consequent reduction of the level of vibrations generated during synchronous operation;
the motor is rendered monodirectional by means of the correct design of the vanes of the impeller, so that the power absorbed by the load in one direction of rotation is greater than the available power of the motor and is smaller in the opposite direction of rotation.
Therefore, by designing the motor and the vanes of the impeller so that the power absorbed by the load in one direction of rotation is greater than the available power of the motor and smaller in the opposite direction of rotation, in the first case the impeller goes out of step with respect to the motor, is halted and automatically reverses its motion, whereas in the second case it is driven normally.
It is thus possible to render the pump monodirectional by utilizing the difference in power between what the motor is able to deliver and the power absorbed by the load in the two directions of rotation (the rotor stops because the power required by the impeller in the wrong direction of rotation is greater than the power that the motor can deliver).
Although this system provides a fundamental advantage with respect to the prior art, it still has limitations, because monodirectionality is ensured only within a flow-rate/head range; accordingly, it is used in applications where the hydraulic working point does not vary beyond certain limits or, in other words, where the characteristic curve of the duct does not undergo significant variations (this is the case, for example, of washing pumps for dishwashers).
In the accompanying drawings FIG. 1 plots, for both directions of rotation of the motor, the power absorbed by the motor as a function of the required flow-rate.
The line A plots the correct direction of rotation, the line B plots the wrong direction of rotation, and the straight line C represents the maximum power that can be delivered by the motor.
The chart shows three flow-rates Q1, Q2 and Q3, which correspond to three working points, and it is clear that only Q1 and Q2 are the flow-rates for which a single direction of rotation is ensured, since the maximum power that the motor is able to deliver (straight line C) is greater than the power required by the impeller when it turns in the correct direction of rotation (line A) and is smaller than the power required by the impeller when it turns in the opposite direction (line B).
For the flow-rate Q3, instead, there is a condition in which both power levels, in both directions of rotation, are lower than the maximum deliverable power and therefore monodirectional behavior is not possible.